Aerodynamic design of the wing-fuselage junction for the high-performance sailplane Mu-31

Abstract

An analysis and design process is described for a new wing-fuselage junction to be applied in the high performance sailplane Mu-31 of Akaflieg Muinchen. First, wind-turmel geometries that were previously tested are analysed with a panel code. The mathematical background is described which enables the interpretation and understanding of the unexpected experimental data. The new design is developed on the basis of one of the previous wind-tunnel geometries, taking construction guidelines and limitations into account. The wing center section and the fuselage are changed iteratively to improve the configuration in potential flow first, i.e. with respect to induced drag. The integral design process is extended with viscous calculations to identify transition and areas prone to separation. The MU-31 is a sailplane prototype aiming at an improved wing-fuselage design described in the present work. With a wingspan of 1.5 meters and camber changing flaps, the MU-31 fulfils all requirements of the FAI 15-Meter Class that is flown at all major championships. To reduce cost and time, existing moulds of the ASW-27 - manufactured by Alexander Schleicher Segelflugzeugbau GmbH in Germany - will be used for the cockpit, the outer wings and the stabilizers. The center section of the wing and the fuselage contain the new approach in wing fuselage design. In the first chapter of this paper, the unexpected outcome of the wind tunnel measurements has been explained. The large differences in drag between the three wind tunnel models can be primarily addressed to different circulation distributions, resulting in different induced drag contributions. It has been shown that the boundary layer on the wing-fuselage juncture is also highly influenced by interference effects creating areas of separation especially on the pylon. With an iterative integral design process taking inviscid- and virous flow into consideration, it is possible to reduce the interference problems. A combination of changing fuselage sections, wing sections and, in particular, wing twist, led to a substantial reduction in induced drag: the final design has an almost elliptical circulation distribution and corresponding minimum induced drag. Viscous flow calculations indicate that separation on the fuselage-pylon or the upper and lower wing side at the juncture will not occur. The actual drag reduction, however, need to be verified by future wind tunnel tests and compared to previous results.